Conventional silicon solar cells, such as crystalline-silicon solar cells, primarily employ a silver-based metallization for the front surface current-collection grid and for the rear surface contacting areas. The silver is usually applied in paste format by screen printing. Conventional silver pastes consist of silver particles and glass frit particles mixed with an organic resin. The organic resins are required as a carrier for the printing process. The glass frit particles soften during heating especially during a firing step to hold the silver particle matrix together with the silicon substrate and to facilitate the formation of a low contact resistance metal contact on the surface of silicon solar cells
Hideki Akimoto in US Patent Application Publication No. 20110223713 describes a method of producing a solar cell electrode, comprising the steps of applying on at least part of a light receiving surface of a semiconductor substrate a conductive paste comprising conductive component, glass frit and resin binder, wherein the conductive component comprises silver particles and core-shell particles in which a metal selected from the group consisting of Pd, Ir, Pt, Ru, Ti and Co is coated on the surface of silver or copper and aids during firing of the conductive paste.
In JP Patent Application No. 2006-295197, examples of electrically conductive particles include metal particles selected from the group consisting of Cu, Au, Ag, Pd, Pt and alloys of Cu, Au, Ag, Pd and Pt .The document describes the case of silicon solar cells in which the electrodes are formed on both sides; the light receiving side paste usually contains as basic components, electrically conductive particles in the form of Ag, binder, glass frit and a solvent.
U.S. Pat. No. 6,322,901 (Bawandi et al.) discloses Group II-VII and III-V semiconductor nanocrystals with core shell structures. The structures disclosed by the author featuring high luminescence efficiency were obtained by forming a semiconductor layer with a band gap greater than the core on the surface of a core nanocrystal.
Methods of synthesizing multishell metallic particles include vapor deposition processes, such as metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), evaporation, sputtering, electroplating, electroless plating, cementation and wet chemistry techniques. Electroplating and vacuum processes are not feasible for commercial purposes due to the lower efficiency of the particles.
US Patent Application Publication No. 20100062154 (Application No. 11603546) discloses a method for preparing a nanoparticle having a coreshell structure by wet chemistry technique. The method comprises (i) preparing a core nanoparticle of a powder form, (ii) dissolving a shell precursor in a solvent to form a shell precursor solution, and then allowing the shell precursor solution to be stabilized at a temperature suitable for the shell precursor to form an overcoat on the surface of a core-nanoparticle. However, such a conventional dropwise method has problems where the stability of the core in such a reaction solvent is easily affected by small changes in reaction conditions such as the temperature, solvent and concentration.
Electrically conductive fine particles and anisotropic electrically conductive materials are disclosed in WO Patent Application Publication No. 2006080289 and US Patent Application Publication Nos. 2006062902 and 2008090092.
Some non-patent literature documents (Schaadt et al., Appl. Phys. Lett. 86, 063106, 2005; Derkas et al. Appl. Phys. Lett. 89, 093113, 2006, Lim et al., J. Appl. Phys, 101, 104309, 2007; F. J. Beck, J. Appl. Phys, 105, 114310, 2009; Rand et al. J. Appl. Phys. 96, 7519, 2004) describe the use of nanoparticles not only on the front but also on the rear end of the active layer of the photo-voltaic devices with the surface plasmon enhanced scattering effect. Plasmonic Au-citrate core shell nanoparticles are synthesized chemically and deposited on the surface of wafer based silicon photovoltaic devices. Standard sodium citrate reduction method has been used for the synthesis of these particles with core shell structure.
US Patent Specification No. 8,304, 362 B2 reports the preparation of coreshell particles as electrocatalysts for oxygen reduction, The core comprises an alloy of as precious metal, whereas the shell consists of a Pt monolayer deposited by underpotential deposition (UPD) (Ref.: J. Zhang, F. H. B. Lima etal. Journal of Physical Chemistry B Letters 2005, 109, 22701-22704.
The catalyst obtained was a Pt X C (X=Au, Ag, Pd; ML=monolayer) with the metal particles comprising an inner core consisting of metal X and a monolayer of platinum in form of a shell on top of it.
U.S. Pat. No. 7,053,021 discloses a method for the preparation of carbon-supported coreshell nanoparticles of 1-3 nm size comprising a platinum-vanadium-iron alloy.
U.S. Pat. No. 4,309,457 discloses a method for producing multilayer coated composite powder which comprises a first step of coating a starting core powder with first metallic ions and a metallic powder for reducing the first ions thereby resulting in a single layer coated powder by cementation reaction; a second step consisting of treating the single layer coated powder either with only second metallic ions other than said metallic ions or with said second ions in the presence of a metallic powder for reducing said second ions to a double layered coated powder by cementation reaction.
As these processes are completely different from current production practices, they cannot be integrated easily into current production lines.
Further art is represented by JP Patent Application Laid-Open No. S64-718 which discloses a method in which palladium is randomly attached to the surface of the copper particles and then the copper powder is put into an electroless nickel plating bath for a predetermined period of time to apply nickel coating of thickness 0.5 μm to the copper powder. This known method has drawbacks in the respect that the electroless nickel plating does not fix strongly to the surface of copper particles and unevenly coating of copper particles with the electroless plated nickel is seen.
Another patent document (JP Kokai Publication 2002-266079) discloses conductive fine particles wherein a base layer made of nickel is formed by electroless plating on the surface of base particles, which are fine particles, and a silver layer is directly formed on the surface of the base layer made of nickel using a cyanogen-free plating liquid which contains a complexing agent such as sodium sulfite or succinimide and a reducing agent such as hydrazine or sodium phosphinate.
This known method has drawbacks. It is not necessary to use copper which easily inter-diffuses with silver and therefore there is no need for the troublesome process of forming a copper layer.